I understand the firing arcs of the ships, but that wasn't what was happening, so let me give this another try.
1. Phase Jump into Enemy gravity well, have TEC Loyalist Titan target SB, Titan and fleet move into range and fire.
2. Titan Lasers target\fire on defense turret.
3. Trade ship moves into range, lasers target\fire on trade ship.
4. Trade ship is destroyed\moves out of range.
5. Titan lasers resume target\fire on defense turret.
Step 3 shouldn't happen, the trade ship is a non-combative, the Titan should have kept the lasers on the defensive turret.